CO and NO Adsorption and Dissociation at the Β-Mo2C(0001) Surface: A Density Functional Theory Study

Adsorption and dissociation of CO and NO molecules at the Mo- and C-terminated β-Mo2C(0001) surfaces has been investigated systematically using density functional theory methods together with cluster models. The calculations yield stable CO and NO adsorption for both surface terminations, suggesting strong adsorbate binding. Molecular adsorption of CO exhibits similar stability for the two terminations, while the molecular NO adsorbate prefers Mo termination over C termination. Computed vibrational frequencies of CO and NO are compared with data from infrared (IR) spectroscopy, allowing a detailed interpretation and assignment of the different features in the experimental spectra. C, N, and O atoms are quite strongly bound at the β-Mo2C surface, where at the Mo-terminated surface, hollow sites are energetically preferred. For the C termination, only oxygen adsorbs near carbon sites, whereas C and N stabilize above Mo substrate atoms or in hollow sites. Dissociative adsorption of NO is energetically preferred over molecular adsorption, while for CO, the two types are energetically similar. Dissociation barriers of adsorbed NO are lower than those for CO, which is consistent with the experimental results. The barrier calculations show also that dissociation prefers the Mo-terminated over the C-terminated surfac